Method for compressing viscous material through openings

ABSTRACT

A novel apparatus for compressing viscous material through openings in a stencil is disclosed. The novel apparatus has a compression head cap which provides a contained environment to direct and to aid the flow of pressurized viscous material through the openings in the stencil.

This is a continuation of application Ser. No. 08/363,806, filed Dec.27, 1994 abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate generally to methods anddevices for depositing viscous materials onto a printed wiring board. Inone aspect, the present invention relates to methods and devices forcompressing viscous materials, such as solder paste, through openings ina perforated substrate, such as a patterned screen or stencil.

2. Description of Related Art

Surface Mount Technology (SMT) involves placing circuit components ontocircuit paths embedded on the upper surface of a printed wiring boardand then soldering the components in place by a process called “reflowsoldering”. Before the circuit component is placed on the printed wiringboard, however, it is desirable to apply solder paste to the area on theprinted wiring board where the component is to be soldered into place.

Conventional methods do exist to deposit (“print”) solder paste ontodesired areas of a printed wiring board by forcing the paste throughopenings in a substrate (e.g., a stencil) placed in intimate contactwith the printed wiring board.

U.S. Pat. No. 4,622,239 describes such a method and device fordispensing viscous materials. The method includes forcing a viscousmaterial from a housing through an opening and depositing it onto astencil between a pair of flexible members (parallel squeegee blades)which depend from the housing on either side of the opening and are incontact with the stencil. The ends of the flexible members are notconnected and remain open ended. The viscous material, accordingly, isnot contained within an enclosed area when it is deposited on thesurface of the stencil. Movement of the housing and the flexible membershorizontally across the stencil causes the trailing flexible member toforce the viscous material through the openings in the stencil. U.S.Pat. No. 4,720,402 describes a similar method and device except that theleading flexible member is raised off of the stencil during movement ofthe housing.

U.S. Pat. Nos. 5,133,120 and 5,191,709 describe methods for fillingthrough-holes of a printed wiring board via a mask with pressurizedconductive filler material by means of a nozzle assembly unit having anozzle tip member. The nozzle tip member, however, is designed only todispense the pressurized conductive filler material through the mask toa single through-hole. The nozzle tip member then “scans” the printedwiring board for a second through-hole to fill. The nozzle tip memberhas a blunt end section which rests on the mask and a circular exit, thediameter of which may be increased or decreased by changing the nozzletip member. The nozzle tip member dispenses the filler material withoutcontrolling unwanted flow of “excessive” filler material back throughthe stencil. Additionally, the nozzle tip member does not define acontained environment where “compression” of the filler material takesplace through the mask followed by the immediate shearing off of thefiller material within that contained environment from the surface ofthe stencil. In fact, the nozzle tip member itself provides no effectivemeans for shearing off filler material from the top of the stencil,rather, after the through hole is filled and filler material “backs up”through the stencil, the nozzle tip member moves forward whereupon the“excessive” filler material is then wiped off by a separate, single,flexible squeegee member which is designed for unidirectional use only.

Unfortunately, these conventional efforts do not provide a containedenvironment for compression of viscous material through holes in astencil and shearing of viscous material within the containedenvironment from the upper surface of the stencil. Reliance uponsqueegee movement to force the viscous material, such as solder paste,through the stencil openings can lead to damage and eventual failure ofboth the squeegee blades and the stencil due to repeated friction. Sinceconventional efforts do not provide a contained environment in whichcompression and shearing is accomplished, waste of the viscous materialis frequently encountered.

Conventional efforts, therefore, (1) fail to maximize the efficiency ofprinting solder paste onto a desired area of a printed wiring board and(2) fail to minimize waste of the solder paste during the printingprocess. A need therefore exists to develop a method for printing solderpaste onto a printed wiring board and a device suitable for usetherewith which overcomes the deficiencies of the conventional efforts.It is accordingly an object of the present invention to provide a novelapparatus for compressing a viscous material through openings in astencil by means of a pressure source. It is a further object of thepresent invention to increase the efficiency of printing viscousmaterial onto a desired area of a printed wiring board and to minimizewaste of the viscous material during the printing process.

SUMMARY OF THE INVENTION

The present invention includes a novel apparatus and method fordispensing viscous material through openings in a stencil. Embodimentsof the present invention include a process herein referred to as“compression printing” wherein pressure is applied to a viscous materialwithin a contained environment defined by a compression head cap so asto compress it through openings in a stencil.

The apparatus of the present invention includes a reservoir containingviscous material which is operably connected to a pressure source. Thereservoir is in fluid communication with a housing which terminates in asubstantially uniform opening defined by a compression head cap formedfrom contiguous walls. During operation of the apparatus, thecompression head cap is placed in contact with a stencil having aplurality of openings therein. The compression head cap and the stencilform a contained environment. The pressure source then applies pressureagainst the viscous material contained in the reservoir forcing it fromthe reservoir into the housing and to the compression head cap. Thecontiguous walls of the compression head cap act to contain and todirect flow of the pressurized viscous material to the top surface ofthe stencil and then through the openings in the stencil.

Other features or advantages of the present invention will becomeapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the detailed description of certain preferredembodiments to follow, reference will be made to the attached drawings,in which,

FIG. 1 is a perspective view of the apparatus of the present invention,partially exploded.

FIG. 2 is a side cross-sectional view of a compression head, acompression head cap, a stencil and a printed wiring board of thepresent invention showing movement of the solder paste through thecompression head, the compression head cap and the openings of thestencil onto the printed wiring board.

FIG. 3 is a bottom perspective view of the compression head cap of thepresent invention, partially broken away.

FIG. 4 is an exploded perspective view of the compression head of thepresent invention showing components of the compression head cap and oneembodiment of a diffuser of the present invention.

FIG. 5 is an exploded perspective view of the compression head of thepresent invention showing components of the compression head cap and asecond embodiment of a diffuser of the present invention.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

The principles of the present invention may be applied with particularadvantage to obtain an apparatus for compressing a viscous materialthrough openings in a stencil, preferred embodiments of which may beseen at FIGS. 1, 2, 3, 4, and 5 which are described more fully below.

FIG. 1 is a partially exploded perspective view of one embodiment of theapparatus of the present invention. As can be generally seen at FIG. 1,the apparatus has a pressure source 2 which is operably connected to areservoir,depicted in FIG. 1 as syringe 4 containing a supply of aviscous material, a compression head 6 and a compression head cap 8.

More particularly, FIG. 1 shows a pressure source depicted as an aircylinder 2 having a piston (not shown) connected to a rod 10 containedin a cylinder housing 12. Pressure inlet 14 allows for the introductionof air pressure at the top of air cylinder 2 thereby displacing thepiston causing piston rod 10 to move downward. The air cylinder 2 ismounted via base 16 to cylinder mount 18 which in turn is removablymounted to casing 20 via a twist socket connector having posts 22 andsocket grooves, one of which is shown at 24. Base 16 has air inlet 26for the introduction of air pressure at the bottom of air cylinder 2thereby displacing the piston causing piston rod 10 to move upward.

Piston rod 10 extends through opening 28 of cylinder mount 18 and isfixedly connected to syringe pusher 30 at contact 32 which in turnengages displacement piece 34 which is movably disposed within syringe 4which contains a viscous material. Displacement piece 34 acts as aplunger and is designed to mate with the interior of syringe 4 to ensureeffective displacement of viscous material with minimal waste. Pressuresource 2 is designed to mechanically meter out viscous material throughoperation of the syringe pusher 30 on the displacement piece 34. Thesyringe pusher 30, displacement piece 34 and syringe 4 are allvertically housed in operative fashion within the cylinder mount 18, thecasing 20 and the syringe housing 36.

The vertical arrangement of the pressure source 2 and the syringe 4containing the viscous material is a preferred arrangement whichadvantageously provides for even and direct pressure in metering out theviscous material onto the top surface of a stencil. It is to beunderstood that pressure source 2 is not limited to an air cylinder ofthe type depicted in FIG. 1, but that other suitable pressure sourcesmay be used by one of ordinary skill in the art based upon the teachingsof the present invention. Such pressure sources include those whichmechanically, electrically, or hydraulically operate a mechanical force,such as a piston rod and displacement piece, to meter out viscousmaterial from a syringe housing or other reservoir which containsviscous material. In addition, pneumatic pressure may be used directlyto force viscous material from a reservoir housing. Also, pressuresource and reservoir configurations other than the verticalconfiguration depicted in FIG. 1 are useful in the present invention.Such configurations include side mounted reservoirs and pressure sourcesor other configurations readily known to those skilled in the art.

The syringe 4 is preferably a disposable unit which can be replaced whendesired by disconnecting cylinder mount 18 from casing 20 via the twistsocket connector, removing the syringe and replacing it with analternate syringe. Examples of disposable syringes useful within theteachings of the present invention include those which are readilycommercially available from Methods Engineering, Vauxhall, N. J. Thecartridges may be purchased prefilled with suitable viscous materials orthey may be purchased empty and then filled with suitable viscousmaterials, such as solder pastes, which are useful within the practiceof the present invention. Useful solder pastes may be readilycommercially available from Alpha Metals, Jersey City, N. J.

Typical solder pastes useful with surface mount technology generallycontain an alloy of tin, lead and silver in various proportions incombination with other useful solder paste metals, viscosity agents,flux and/or solvents depending upon the desired use of the solder paste.Solder pastes useful in the present invention will become apparent toone of ordinary skill in the art based upon the teachings herein.

The syringe housing 36 is mounted to a housing referred to herein as acompression head generally depicted at 6 in FIG. 1 and shown in across-sectional side view in FIG. 2. The syringe 4 has flange opening 38which is inserted into and mates with first opening 40 of compressionhead 6 which is described hereafter with reference to both FIGS. 1 and 2and FIG. 3 which is a bottom perspective view of the compression headcap 8, partially broken away. The compression head 6 terminates in asubstantially uniform second opening 42 which is defined by compressionhead cap 8. The compression head cap 8 is formed from contiguous wallswhich define a volume 44 within compression head cap 8. The contiguouswalls may be either unitary or formed from separate elements and aredesigned to contact stencil 46 to provide a uniform and substantiallyflush union with stencil 46 at the point of contact. As can be seen inFIG. 2, stencil 46 has openings 48 and is placed in an operablerelationship with a printed wiring board 50. The stencil 46 may beplaced in intimate contact with printed wiring board 50 or, as shown inFIG. 2, it may be placed a distance above printed wiring board 50 suchthat pressure from the compression head 6 forces the stencil 46 intocontact with printed wiring board 50. Although stencil 46 is shown incross-section, it is to be understood that openings 48 may have anydesired orientation on stencil 46. Further openings 48 may differ insize depending upon the area of the printed wiring board 50 to beprinted with the viscous material. The compression head cap 8 andstencil 46 together form a contained environment for the viscousmaterial during operation of the apparatus of the present invention.

As can be seen in FIG. 1, cross bar mount 52 is attached to syringehousing 36 via flange 54. Cross bar mount 52 is also attached to amechanism (not shown) for horizontally displacing the apparatus of thepresent invention along the stencil 46.

The compression head 6 is preferably formed from metal, such as iron orstainless steel or other material suitable for use with pressurizedviscous material. The compression head 6 has top surface 56 which servesas the base to which the syringe housing 36 is attached. Side surfaces58 and 60 extending from top surface 56 slope away from each other asdepicted in FIG. 1 to define an increasing length of compression head 6.Front and back surfaces 62 and 64 are contiguous with side surfaces 58and 60 and slope toward each other as depicted in FIG. 2 to define adecreasing width of compression head 6. The side surfaces join with thefront and back surfaces to define a tapered interior chamber 66 as shownin FIG. 2 which acts to restrict flow of viscous material throughcompression head 6.

Interior chamber 66 terminates in generally rectangular exit 68.Compression head 6 preferably acts as a nozzle to direct and constrictthe flow of viscous material through generally rectangular exit 68. Thecompression head cap 6 defines a volume 44 surrounding the rectangularexit 68 and into which viscous material flows after exiting the interiorchamber 66 of compression head 6. As shown in FIG. 2, the volume 44 ispreferrably a separate chamber into which the viscous material flowsafter exiting the interior chamber 66 via rectangular exit 68.

The compression head cap 8 defines a generally rectangular opening 42which is to be contacted with stencil 48. The compression head cap 8acts to contain and direct the flow of viscous material to the stencil46. In an alternate embodiment, it is to be understood that the interiorchamber 66 may terminate directly into compression head cap 8 withoutthe need for rectangular exit 68 or volume 44.

As can be seen in FIG. 4, the compression head 6 has two half sections70 which are fixedly connected by screws (not shown) via screw holes 72.The bottom side section 74 of each half section 70 is provided withledge area 76 to engage the compression head cap 8. As depicted in FIGS.2, 3 and 4, compression head cap 8 has rectangular blades 78 and endcaps 80, which define generally rectangular opening 42. Blades 78 areeach fixedly mounted to a corresponding ledge area 76 of front surface62 and back surface 64, respectively, by means of correspondingrectangular blade holders 82 and screws (not shown) via screw holes 84.Blades 78 each extend along substantially the entire length ofcorresponding ledge area 76. End caps 80 are attached to a correspondingledge area 76 of bottom side section 74 via corresponding cap mounts 86and screws (not shown) via screw holes 88. The end caps 80 arecontiguous with blades 78 and together form the compression head cap 8.As can be seen more clearly in FIG. 2, blades 78 parallel the slope ofcorresponding front and back surfaces 62 and 64, and are, therefore,seen to be angled inward relative to the interior chamber 66 ofcompression head 8.

Blades 78 are preferably thin and formed from rigid material such asiron or stainless steel. End caps 80 are preferably formed from aflexible substance such as polyurethane to avoid damage to the stencilduring operation of the apparatus of the present invention. Cap mounts86 and blade holders 82 are formed from any solid material capable ofsecuring the corresponding end cap or blade.

While the compression head cap 8 is depicted in FIGS. 2, 3, and 4 asbeing formed from integral parts, it is to be understood thatcompression head caps having a unitary structure are within theteachings of the present invention. Such unitary compression head capsare formed from a single rectangular shaped unit and are designed toencircle the ledge area 76 of the compression head 6 or otherwiseoperatively engage compression head 6. Such compression head caps may beeither fixed or removably mounted to the compression head and may havevarious sizes of opening 42.

The compression head 6 and compression head cap 8, in combination withthe pressure source 2 and syringe 4 advantageously provide a verticaldown force to move the viscous material evenly and directly to thestencil. The compression head cap 8 of the present inventionadvantageously provides a contained environment to direct and to aid inthe extruding of pressurized viscous material through openings in thestencil. The extruded viscous material is then deposited on the patternof the printed wiring board. The apparatus of the present inventionprovides for very high speed printing capability while maintaining printdefinition and reduced cycle time. Waste of viscous material isminimized due to the contained environment provided by the compressionhead cap 8. The length of the compression head cap 8 allows forsimultaneous compression printing through a plurality of openings 48 instencil 46. Furthermore, the trailing blade 78 relative to the directionof operation advantageously operates to shear off the viscous materialcontacting the stencil within the compression head cap 8 when theapparatus of the present invention is horizontally disposed across thestencil. The blades 78 are rigid and angled to advantageously achieve asmooth shearing of the viscous material. The compression head 6 andcompression head cap 8 are advantageously rectangular in shape so thatthey may operate over a significant area of the stencil with each pass.Additionally, given the dual blade design of the compression head cap 8,the apparatus of the present invention may operate in both the forwardand reverse directions thereby improving the efficiency of thecompression printing process of the present invention.

As can be further seen in FIGS. 2 and 4, compression head 6 has diffuser90 which is fixedly mounted within interior chamber 66. Diffuser 90 hasa plurality of diffuser plates 92 which are horizontally disposed withininterior chamber 66 via grooves 94. Each diffuser plate 92 has a seriesof openings 96 through which viscous material is to flow. Each opening96 may be either circular or oblong and decreases in average size as thediffuser plates progress from the first opening 40 to the compressionhead cap 8. The openings 96 also increase in number as the diffuserplates progress from the first opening 40 to the compression head cap 8.The diffuser 90 advantageously serves to break up the flow of viscousmaterial and evenly and uniformly distribute it from side to side of therectangular exit 42. The diffuser 90 may also serve to reduce thevelocity of the viscous material flowing through the compression headand increase the static pressure of the viscous material which aids inthe compression printing process.

FIG. 5 shows an alternate embodiment of a diffuser 90 useful in thepresent invention. The diffuser 90 has a plurality of diffuser islands98 which are horizontally disposed in rows within interior chamber 66.The diffuser islands 98 may be fixedly installed within the interiorchamber 66 or they may be molded directly within the interior chamber66. As with the diffuser 90 of FIG. 4, each diffuser island 98 acts tobreak up the flow of the viscous material and uniformly and evenlydistribute it from side to side of rectangular exit 42. The diffuserislands may be either circular or oblong and decrease in average size asthe rows progress from the first opening 40 to the compression head cap8. The diffuser islands 98 also increase in number as the rows progressfrom the first opening 40 to the compression head cap 8. The diffuserislands 98 of the present invention are advantageous in that theyprovide for ease of fabrication of the compression head and ease ofcleaning.

Operation of the apparatus of the present invention is now described asfollows with reference to FIGS. 1 and 2. When compression printingaccording to the teachings of the present invention, the compressionhead cap 8 of the apparatus of the present invention is brought intocontact with the top surface of stencil 46 which forces the stencildownward until it is in intimate contact with the printed wiring boardbelow as shown in FIG. 2. The apparatus is then moved in a horizontaldirection, as shown in FIG. 2, across the stencil 46.

During movement of the stencil, pressure source 2 acts on syringe 4 toforce viscous material 100 from the syringe 4 into the interior chamber66 of compression head 6 where it is diffused by diffuser 90 anddirected to rectangular exit 68. The viscous material then enters volume44 of compression head cap 8 which provides a contained environment viablades 78 and end caps 80 to direct the pressurized viscous materialunder pressure to the top surface of stencil 46. The viscous material isthen extruded through openings 48 in the stencil 46 over which thecompression head cap 8 travels. The extruded viscous material 100 isthereby printed on the printed wiring board 50. Movement of thecompression head cap 8 across the stencil surface causes the trailingblade 78 which is angled inwardly relative to the interior chamber 66 toshear off the viscous material from the top surface of stencil 46. Oncethe apparatus has traversed the length of the stencil, the apparatus maysimply reverse its direction and continue the compression printingprocess since the compression head cap 8 has dual blades 78 toaccomplish the shearing process in either direction of movement.

Operating variables of the apparatus of the present invention, such asrun speed and pressure, may be adjusted to accommodate either viscousmaterials having a wide range of viscosities or stencils with holeshaving a wide range of diameters. The following data in Table 1 isrepresentative of the parameters at which the apparatus has successfullyoperated. Print speed is measures in inches per second, air pressure ismeasured in pounds per square inch, viscosity of the solder paste ismeasured in centipoises per second, stencil apertures are measured ininches, and the particle sizes of the solder pastes used are between10-37 microns.

TABLE 1 Print Speed Viscosity Aperture (inches/sec.) Air Pressure (cps)(inches) Low High (psi) Low High Low High 0.94 1.26 20 850K 1.0M0.0055 >0.025 1.45 1.70 20 850K 1.0M 0.0055 >0.025 2.27 2.31 20 850K1.0M 0.0055 >0.025 3.10 3.89 30 850K 1.0M 0.0055 >0.025 4.20 4.77 30850K 1.0M 0.0055 >0.025 5.98 6.62 40-50 850K 1.0M 0.0075 >0.025 6.697.23 50-60 850K 1.0M 0.0075 >0.025 7.70 8.00 50-60 850K 1.0M 0.0075>0.025

As indicated by the above data, the apparatus of the present inventionsuccessfully operated over a wide range of print speeds, air pressuresand stencil openings. The compression printing method disclosed hereinadvantageously provides for quicker print speeds, better quality ofprinting, and less waste of solder paste material than is encounteredwith conventional printing methods.

It is to be understood that the embodiments of the invention which havebeen described are merely illustrative of some applications of theprinciples of the invention. Numerous modifications may be made by thoseskilled in the art without departing from the true spirit and scope ofthe invention.

What is claimed is:
 1. A method for compressing a viscous materialthrough openings in a stencil comprising, the steps of, (a) directing aviscous material into a housing terminating in a compression head capconfigured to achieve a substantially flush union with the stencil toprovide a contained pressurized environment for the viscous material soas to force the viscous material through the openings in the stencil andwherein the compression head cap comprises front and back blades; (b)placing the compression head cap in substantially flush union with thestencil having openings therein; and (c) applying pressure against theviscous material sufficient to force it from the housing via thecompression head cap so that pressure in the contained pressurizedenvironment forces the viscous material through the openings of thestencil.
 2. The method of claim 1 further comprising the step of movingthe housing across a top portion of the stencil so as to cause thecompression head cap to shear viscous material from the top portion ofthe stencil.
 3. The method of claim 2 wherein the stencil is in intimatecontact with a printed wiring board.
 4. The method of claim 3 where thefront and back blades are angled inward relative to the containedpressurized environment.
 5. The method of claim 4 wherein at least oneof the blades angled inward relative to the contained pressurizedenvironment shears viscous material from the top portion of the stencil.6. The method of claim 5 wherein opposing ends of the blades areconnected by end caps.
 7. The method of claim 6 further comprising thestep of diffusing the viscous material by a diffuser fixedly disposedwithin the housing.
 8. The method of claim 7 wherein the diffusercomprises rows of diffusing islands of successively smaller size.
 9. Themethod of claim 7 wherein the diffuser comprises a series of parallelplates having successively smaller openings.
 10. The method of claim 7wherein the housing is progressively tapered to restrict flow of theviscous material through the housing.